1. Field of the Invention
The present invention relates to an improvement in a chuck which is adapted to be attached to the spindle of a machine tool so as to grip a rotary cutting tool such as a drill or an end mill.
2. Description of the Prior Art
The conventional chuck using claws for gripping either a tool or an adaptor for holding a tool has such a localized gripping portion that its gripping force is insufficient to hold the tool and, still worse, its claws may damage the tool. In order to help alleviate these problems, there has been developed and widely used the so-called "needle-roller type chuck", as is disclosed in German Pat. No. 931,452, for example. In the chuck of this type, a cylindrical chuck barrel having an outer peripheral surface tapered into a conical surface is made integral with a chuck body. A clamp ring is rotatably fitted over the chuck barrel. The clamp ring has an inner conical surface extending in parallel with the outer peripheral surface of the chuck barrel. There are interposed between the chuck barrel and the clamp ring a multiplicity of needle rollers which are rotatably held in a retainer. These needle rollers are in contact with the outer peripheral surface of the chuck barrel and the inner peripheral surface of the clamp ring such that their axes are so inclined with respect to the axis of the chuck body as not to intersect therewith. Thus, when the clamp ring is rotationally fastened, the needle rollers spirally revolve along the conical surface while rotating about their own axes so that the chuck barrel may be pushed and contracted all over its surface thereby gripping the tool without damaging it or causing it to fall. If the clamp ring is rotationally loosened, on the other hand, the needle rollers are retracted in the direction toward the leading end of the chuck barrel so that the chuck barrel may be restored to allow the tool to be removed without difficulty.
However, since the conventional needle-roller type chuck thus far described has its gripping portion, i.e., its chuck barrel thinned to permit large deformation, its rigidity is so low as to result in undesirable chattering or rapid vibration at the waist of the chuck, especially, at the base portion of the chuck barrel when the cutting operation is performed on heavy duty material. In order to compensate for such insufficient rigidity, another needle-roller type chuck has been conceived which is thicker and which uses double rows of needle rollers, as is disclosed in U.S. Pat. No. 4,021,051 issued on May 3, 1977. However, since the construction is double in nature, ensuring the concentricity of the chuck is a problem, in addition, the construction is unavoidably complicated.